It is well recognized that amorphous polypropylene (aPP), even at a very high molecular weight (e.g. M.sub.w &gt;1000,000 g/cc) is a soft, rubbery, gel-like material which possesses very low crystallinity and therefore poor physical properties. Because of its poor mechanical strength, this material has found few practical uses compared to isotactic polypropylene (iPP), which has crystallinity and therefore better mechanical properties.
Individual polyolefins having certain characteristics are often blended together in the hopes of combining the positive attributes of the components. Typically, however, the result is a blend which displays a weighted average of the individual properties of the individual resins.
For example EP 0 527 589 discloses blends of flexible low molecular weight amorphous polypropylene with higher molecular weight isotactic polypropylene to obtain compositions with balanced mechanical strength and flexibility. These compositions show better flexibility compared to the isotactic polypropylene alone, but the elastic recovery properties are still poor.
U.S. Pat. No. 5,539,056 discloses polyolefin compositions comprising a blend of amorphous poly-alpha-olefin having a weight average molecular weight (M.sub.w) of at least about 150,000 and a crystalline poly-alpha-olefin having an M.sub.w of less than about 300,000 and less than that of the amorphous poly-alpha-olefin. These compositions were produced by polymerizing alpha-olefin in the presence of two different cyclopentadienyl transition metal compounds or by producing the polymers independently and subsequently blending them together.
EP 0 366 411 discloses a graft polymer having an EPDM backbone with polypropylene grafted thereto at one or more of the diene monomer sites through the use of a two-step process using a different Ziegler-Natta catalyst system in each step. This graft polymer is stated to be useful for improving the impact properties in blended polypropylene compositions.
Although each of the polymers described in the above references has new and interesting properties, there remains a need for new compositions offering other new and different balances of mechanical properties controllably tailored for a variety of end uses. It would be desirable to find a composition that is very strong yet having both good flexibility and elasticity characteristics. It would further be desirable to produce such a composition with a minimum of processing steps.